1. Field of the Invention
This invention relates in general to a sports racquet string and more specifically involves a string configuration that imparts more spin on the ball and to a method of manufacturing such an improved string.
2. Background of the Invention
The traditional and most popular cross-section of a sports racquet string is round. Such strings are made typically from natural gut (animal fiber) or from synthetic material, such as nylon. Conventionally, strings are constructed by twisting many fine filaments of these materials together, with or without a center filament, into a round core strand and then by passing the core strand through a round die to apply an outer layer coating.
In general, it is desirable that a string exhibit small damping, that is low energy loss and high resilience, and good elasticity, that is a low modulus of elasticity. These elements contribute to the playability of the string. It is also desirable that the string be sufficiently durable.
The diameter of the string is very important as it affects the durability and playability of the string. Generally, thin strings have superior playability. Thin strings exhibit high resilience and good elasticity, and they maintain longer contact with the ball for greater control. However, thin strings may stretch and are more easily broken. On the other hand, thick strings are stronger and more durable but lack the playability of thin strings.
An additional important characteristic of a string is its ability to impart spin on the ball. For example, in the game of tennis, a player standing behind the baseline would have to have a height of about six foot seven to see any of the opponent's court without looking thru the net. This means that most hard-hit balls passing over the net and not having forward spin will land out of bounds over the opponent's baseline. Ball spin affects the ball's flight characteristics. When a ball leaves the racquet string bed spinning forward, i.e. rotating forward on top, it's flight path will tend to curve downward, and it will land earlier and bounce higher. With good top spin, a player can hit a given ball much harder and still have the ball land in. When a ball leaves the racquet string bed spinning backward, i.e. rotating forward on bottom, its flight path is flatter; it will tend to land further and bounce lower. Thus, if the player can control spin, the player can control to some degree the trajectory of the ball to advantage.
Again, string characteristics largely determine the amount of spin that can be imparted on the ball. As previously mentioned, the amount of string elongation and resilience determines the amount of time the strings are in contact with the ball. Generally, the thinner the string, the greater the contact time. When the ball impacts on the racquet face, the ball remains in contact with the string bed for about three to five thousandths of a second. During this time, the player is able to impose more control over the direction of ball return and is able to impart spin to the ball to control its flight characteristics.
To put spin on the ball, the ball is struck with the racket face at an angle to the flight path and the racket face is moved in the plane of the face. Increasing the friction between the strings and the ball has been thought to enhance imparting spin on the ball.
Synthetic fiber strings, in particular, are excessively smooth in their outer surface and tend to slip over the ball. Many measures have been taken to enhance friction including roughening the outer surface of the string such as by grinding with abrasives, surface coating the string with frictional or rubbery substances, twisting or braiding fiber multifiliments, and winding of silk yarns around the string core.
Synthetic strings treated in the above-described manners tend to have poor dimensional stability and are reduced in strength and elasticity resulting in tension loss during play. Further, some are inferior in durability because they exhibit surface aberrations, wearings or breakages due to degredation of the resins, and abrasion, peeling or denaturing of the treating substances. Moreover, since the above-mentioned treatments constitute additional steps in manufacture, there is an increase in production costs.
Another proposed method to increase string friction is the use of string of polygonal cross-section whereby the sharp corners resulting at the juncture of the faces of the polygon are the spin enhancing portions. Two types of polygonal-shaped string have been proposed. Reta, U.S. Pat. No. 4,805,393 proposes the use of a multi-sided cross-sectional configuration string; Wells, U.S. Pat. No. 4,860,531, proposes a polygonal coating over a round central core.
Traditional round strings may have a thin, evenly distributed coating around the core to provide protection to the core strands which provide the tensile strength and playability of the string.
As two round strings cross over one another, due to the very small contact area they weaken one another by indenting one another and by cutting one another in a sawing action as the strings move relative to one another during play.
Therefore, it is desirable to have an improved sports racquet string having much better spin-imparting characteristics than a conventional round string and which achieves this without significant loss in playability.
It is further desirable, that such a string reduce the weakening characteristics of string cross-over and therefore be more durable than conventional strings.